1. Field of the Invention
The present invention relates to a method and means for continuously contact-coating one side only of a ferrous base metal strip with a molten coating metal, and more particularly to such a method and means whereby the strip need not be submerged in the bath of molten coating metal.
2. Description of the Prior Art
The method and apparatus of the present invention may be used to produce a ferrous base metal strip provided on one side only with a coating of any appropriate hot-dip coating metal such as, for example, zinc, zinc alloy, aluminum, aluminum alloy, terne, lead and the like. While not intended to be so limited, for purposes of an exemplary showing the method and apparatus of the present invention will be described in terms of their use in the production of a ferrous base metal strip coated one-side only with zinc or with aluminum.
In recent years there has been a growing demand for a ferrous base metal strip coated with a protective metal on one side only, as for example a steel strip which has been galvanized on one side. Such a product is particularly useful in industries such as the automotive, appliance and building panel industries. The galvanized side of such a product demonstrates excellent corrosion resistance while the uncoated side is characterized by excellent paintability and can readily be welded by spot welding techniques or the like. In instances where corrosion protection is required on only one side of the product, it will be understood that a one-side coated product will provide a considerable savings of the coating metal.
Prior art workers have devised a number of ways in which to produce a one-side coated ferrous base metal strip. In accordance with one procedure, the ferrous base metal strip is coated one-side with a "stop-off" (i.e., a barrier layer, non-wetting to the coating metal). The strip is conventionally hot-dip coated. Thereafter, the barrier layer is scrubbed off or otherwise removed.
United States Letters Pat. No. 3,383,250 teaches a process wherein the metal strip is appropriately cleaned on both sides, brought to coating temperature and then is caused to be oxidized on one side only. The strip is thereafter caused to pass through a bath of molten coating metal which adheres to the unoxidized side only.
In accordance with another method, the strip is hot-dip coated on both sides with as much as possible of the coating on one side being removed by an air knife or jet. The remainder of the coating metal on the jetted side is then removed by an electrolytic deplating process.
Finally, electrolytic coating has been practiced to provide a one-side coated product. To this end, the strip to be coated passes about a roll partially submerged in an electrolyte. The exposed side of the strip has a metallic coating deposited thereon, while the other side of the strip remains uncoated, being protected by the roll about which it passes.
While these various prior art practices may produce acceptable products, they are characterized by certain deficiencies. In general, the prior art practices are expensive, requiring more steps than ordinary hot-dip coating and using extensive specialty equipment. Presently used masking techniques produce an uncoated surface of marginal quality for high-finish painting applications.
Prior art workers have used a hot metal meniscus to fully coat tubes and bars as taught in German Pat. No. 2,406,939. The process taught in this reference would not, however, be applicable to one-side coating of a ferrous base metal strip.
The method and apparatus of the present invention enable the rapid and continuous contact-coating of one side only of a ferrous base metal strip with a molten coating metal. Coating thicknesses can be controlled as in conventional two-side hot dip coating processes. No in-metal roll assemblies are required, eliminating accrued materials and maintenance problems. The present invention is cheaper and easier to practice than previously used commercial one-side coating methods. Existing in-line anneal type continuous coating lines can be easily and inexpensively modified to produce a one-side coated product in accordance with the present invention and, in fact, with provision for equipment interchangability the same line can be used to produce both a one-side and a two-side coated product as desired. Product quality is superior to that produced by other hot dip methods with respect to both the coated and the uncoated surfaces.